Lifting assemblies such as those found on cranes and other material handling systems may include a crown block and a travelling block. Generally, the crown block is stationary relative to the supporting structure, which could be a crane boom, derrick, a bridge, a trolley, or some other aspect of the material handling system. In contrast, the travelling block may be movable generally upward and downward relative to the crown block by paying out or hauling in cable or rope. Each of the crown block and travelling block may include one or more sheaves and wire rope may be reeved around the sheaves between the crown block and the travelling block to create a block and tackle arrangement. The sheaves in the blocks may have grooves in them for controlling the position of the wire rope as it engages the sheave.
In the context of a drill rig, such as an oil drill rig, the crown block may be supported by a derrick and the traveling block may be suspended below the crown block and used for lifting and/or supporting tubulars in the well bore. A winch or drawworks may be connected to one end of the wire rope and may be used to raise and lower the travelling block. In some operations, such as when new sections of pipe are being lifted into place for example, the travelling block may be pulled horizontally and out from directly below the crown block. This may create a fleet angle between the wire rope and the plane of the sheave on either or both of the travelling and crown blocks. As may be appreciated, this fleet angle may cause the wire rope to hang up on the rim or lip of the sheave as it enters the sheave such that the wire rope intermittently slips into the groove rather than continuously feeding into the groove. This hang up and slippage can cause wear on the sheave and/or the rope leading to premature failure and replacement costs.